According to a known method for manufacturing stiffened panels the stringers are manufactured separately from the outer skin. A series of plies of preimpregnated material are rolled and then cut to obtain a series of strips. Each strip is subjected to a hot-forming treatment which provides them with a cross-section of predetermined shape. An element thus shaped is assembled together with another shaped element so as to obtain a wet stiffening element. The stiffening elements thus obtained are subjected to a curing cycle in an autoclave.
The polymerized or “pre-cured” stringers are then assembled together with the wet skin; the assembly thus obtained is subjected to a second, final, curing cycle in an autoclave.